Power supplying device for the operation of a gas discharge container

ABSTRACT

A power supplying device for the operation of a gas discharge container for the treatment of metallic workpieces. For the provision and the adjustment of the operating voltage the power supplying device comprises a transformer, a thyratron-or thyristor full-wave rectifier located in the secondary circuit and an ignition control device for the ignition of the rectifiers. The power supplying device further comprises a device monitoring the discharge and in response to which the ignition control device further serves as a quick-action switch interrupting the discharge during a number of periods. An inductance is located in the primary circuit having such design that the increase of the operating voltage following every zero passage is retarded. Resulting is an extension of the interruption of discharge occuring at every zero passage of the operating voltage causing minor discharge disturbances on the workpiece surface to decay.

United States Patent 11 1 1111 3,914,575

Eichler Oct. 21, 1975 [5 POWER SUPPLYING DEVICE FOR THE 3,593,105 7 1971Brohaugh 321 40 x OPERATION OF A GAS DISCHARGE 3,737,763 6/1973 Chadwick321/40 X CONTAINER 3,746,966 7/1973 Tiirok et a1.

3,781,508 12/1973 Dauer et a1 219/121 P [75] Inventor: Walter Eichler,Cologne, Germany [73] Assignee: Elektophysikalische Anstalt BernardPrimary Truhe g h Vaduz, Liechtenstein Assistant ExaminerG. R. PetersonFl d F b 6 1974 Attorney, Agent, or FirmBacon & Thomas [21] Appl. N0.2439,872 57 ABSTRACT A power supplying device for the operation of a gas1 Foreign Application Priority Data discharge container for thetreatment of metallic Feb. 19, 1973 Switzerland 2357/73 workpieces. Forthe provision and the adjustment of the operating voltage the powersupplying device com- [52] US. Cl 219/121 P; 315/127; 323/123; prises atransformer, a thyratron-or thyristor full-wave 321/40 rectifier locatedin the secondary circuit and an igni- [51] Int. Cl B23k 9/00 tioncontrol device for the ignition of the rectifiers.

[58] Field of Search..... 219/121 P, 131 R, 131 WR; The power supplyingdevice further comprises a de- 315/111.1, 111.2,111.3,111.4, 111.5, vicemonitoring the discharge and in response to 111.6, 111.7, 111.8, 111.9,136, 127, 119; which the ignition control device further serves as a323/123, 9, 20, 4; 321/40 quick-action switch interrupting the dischargeduring a number of periods. An inductance is located in the [56]References Cited primary circuit having such design that the increase ofUNITED STATES PATENTS the operating voltage following every zero passageis 2 508 954 5/1950 Latour et al 32) X retarded. Resulting is anextension of the interruption 323/123 X of discharge occuring at everyzero passage of the op- 3,205,426 9/1965 Mills 321/40 x erating voltagecausing minor discharge disturbances 3,270,272 8/1966 Kurimura 321/40 xo the workpiece surface to decay.

3,579,029 5/1971 Spescha 315/127 3,582,755 6/1971 Liss et a1 321/40 x 3Clams, 2 Drawmg Flgures 3,189,747 6/1965 Hoff, Jr

- REGULATOR CONTROL 24 AUNIT 12 2 J 15 liq 11 30 3 p 22 u ,wORK .1 BPIECE 11/ 'MONITORING/ DEVICE CONTAINER U.S. Patent 0a. 21, 19753,914,575

Fig.1

REGULATOR CONTROL 24/ UNIT MONITORING/ DEVICE CONTAINER POWER'SUPPLYINGDEVICE FOR THE OPERATION OFA GAS DISCHARGE CONTAINER The presentinvention relates to a power supplying device for the operation of a gasdischarge container for thetreatment of metallic workpieces,particularly for the operation of an ionitriding furnace, comprising atransformer and'rectifiers for the provision of theopcrating voltagefrom the alternating or three-phase current supply," with a continuouslyoperating regulating devicefor the operating voltage, a monitoringdevice for the discharge process during the process, and with aquick-acting 'switch for'the brief interruption of operation when themonitoring device responds.

Several forms of such power supplying devices are known, by way ofexample those using a variable voltage transformer as a continuouslyoperatingregulating device or also grid rectifiers with a regulatingdevice for their control grid. With'all such current supplying devicesdesigned for the operation'of gas discharge processes it is of greatimportance that a quick-response monitoring device is provided whichcauses a quick brief disconnection of the'operating voltage whenirregularities in the discharge occur and which disconnection lastsanurnber of periods. Without such a brief disconnection'a disturbance inthe discharge process, by way of example'a tinygas eruption from thesurface of the metallic workpieces treated, would as is well known causea transition of the gas discharge into an arc discharge, which mightcause damage to the metal surface. Particularly in the operation ofionitriding furnaces with an electrical high-intensity gas discharge,which are employed on an industrial level itcannot be avoided during thestarting process at the beginning of the 'nitriding operation that suchimperfections in the glow discharge frequently occur which necessitatebrief quick disconnection of the glow discharge process. By way ofexample this starting process and its control by brief interruption ofthe operation is disclosed in U.S. Pat. No. 2,884,511.

The monitoring device there described, however, is only one of the manyknown systems for monitoring the operating condition of gas and glow'discharge processes, and further types of such monitoring devices whichhave provided satisfactory and are disclosed for example in U.S. Pat.No. 3,579,029.

In operating a glow or gas discharge with a rectified alternatingvoltage from a single-phase alternating voltage network it has proved tobe of advantage for the discharge to be briefly switched off beforeeveryzero passage of the operating voltage in order to recommence only duringthe subsequentrise of the operating voltage after reaching' theso-called ignition voltage value of the discharge. These briefinterruptions of the discharge process which occur at double thefrequency of the alternating 'voltage employed, however, are generallyso shortthat,:=in the presence of the .glow discharge, the causeofdisturbancehas not sufficiently decayed and thus stillremains when thedischarge process is resumed. YY

The present current supplying device in which the alternating currentisrectified to provide the operating voltage constitutes a particularlysimple and .inexpensive design 'which'employs the effect of the-briefinterruptionof the discharge atevery zero' passage of the rectifiedoperating voltage while avoiding the-aforementioned disadvantages. a a IThe power supplying device according to the invention is characterizedby the combination of:

a. a full-wave rectifier located in the secondary circuit of atransformer comprising thyratron or thyristor components, an ignitioncontrol device for the continuous adjustment of the phase position forthe purpose of controlling the operating voltage and by the simultaneoususe of this ignition control device as said switch when said monitoringdevice responds,

b. an inductance located in the primaray circuit of the transformer, I

0. design of said inductance influencing the phase position of thesecondary voltage applied to the rectifiers so that the increase of theoperating voltage following every zero passage is retarded until minordischarge irregularities on the workpiece surfaces have decayed.

One embodiment of this invention will now be described in greater detailwith reference to the drawing in which F IG. 1 shows a simplifiedcircuit schema of the power supplying device supplying an ionitridingfurnace, and

FIG. 2 shows the diagram of the curve of the operating voltage plottedagainst time.

In the embodiment shown a metallic underpressure container 10 with a gasinlet 11 and a gas outlet 12 accommodates the workpiece 13 which is e.g.to be ionitrided by means of a glow discharge in a nitrogenousatmosphere. The metallic workpiece 13 is connected, via a metalliccoupling member 14, to the inner conductor of an insulated currentlead-in 15 shown diagrammatically and of which the exterior connection16 represents the negative pole of the power supplying device. The otherpole 17 of the power supplying device is connected to the metalliccontainer 10 and ground. The operating voltage U, is applied between theconnections 16 and 17 of which the variation in time is diagrammaticallyshown in FIG. 2.

The voltage U, is applied via the voltage divider formed by theresistances l8, 19 to the monitoring device 20. Also the voltage drop atthe series resistance 21- in the supply line to the connection 17 isapplied to the device 20. The function of this monitoring devicerequires no detailed explanation since it is described in detail in U.S.Pat. No. 3,579,029.

Connected to the monitoring device 20 is a control unit 22 whichdetermines, in accordance with the known phase shifting method, theswitch on point of the thyratron or thyristor rectifiers 28, 29, 30, 31during each half period. This point can be manually set by the regulator23 diagrammatically indicated. The rectifiers 28, 29, 30, 31 constitutea full-wave rectification for the alternating voltage of the secondaryside 27 of the transformer 26.

This device which is known in itself functions as follows: As soon asthe monitoring device 20 responds because of a major irregularity in theglow discharge on the workpiece 13, the control unit 22 receives acontrol impulse. Thereupon the control unit 22 causes the alternatingvoltage in the secondary circuit to remain interrupted after the zeropassage next following during the time of a predetermined number ofperiods as mentioned before. The starting is effected again only by asecond control impulse which is automatically supplied by the monitoringdevice 20 after a delay time which can there be adjusted.

It was found that it is of great advantage to provide an inductance, forexample an iron choke coil 24, in the primary circuit 25 of thetransformer 26, between the transformer and the power supply. Filteringor suppression means for suppressing high-frequency interference by therectifiers 28, 29, 30, 31 may be used in addition. The action of theinductance 24 will be described below,

In principle, the power supplying device according to FIG. 1 describedabove would be entirely suitable, also without the iron choke coil, forthe perfect operation of a glow and gas discharge process of the typedescribed since the monitoring device' 20, the control unit 22 and theadjustable rectifiers 28, 29, 30, 31 ensure that, when disturbances inthe discharge process occur, the operating voltage U remains off afterthe next zero passage during the time of a predetermined number ofhalf-waves until the rectifiers 28, 29, 30, 31 ignite again depending onthe adjusting of the monitoring device 20. Experience has shown,however, that by way of example in the industrial treatment ofworkpieces, particularly the simultaneous treatment of a plurality ofdifferent workpieces in a common discharge container, the number ofarising disturbances is so high, particularly during the startingprocess at the beginning of the ionitriding operation, that this processmay extend over many hours owing to the frequency disconnections.However, the utilization of the ionitriding furnace involved isundesirably reduced by the greatly extended starting period, whichnecessitates the installation of additional ionitriding facilities wherean ionitriding operation has a predetermined capacity. The present powersupplying device with the control unit 22 and the inductance 24 providedin the primary circuit removes this undesirably high extension of thestarting process without involving defects in the workpieces treated.

As indicated in the diagram of FIG. 2, the discharge process isdiscontinued at point 33 before the zero passage of the operatingvoltage U, after the discharge ignition voltage U is negativelyexceeded. This discharge process will normally start again as theoperating voltage U increases after reaching the ignition voltage U ofthe discharge at point 34. The period t, of discharge interruption atthe zero passage of the operating voltage U normally lasts only amillisecond or less and is too brief to enable defects on the workpiecesurfaces to decay so that, when the operating voltage U has passed thepoint 34, such defects remain active and will result in the response ofthe monitoring device 20 and thus in said predetermined number ofperiods of disconnection of the discharge container 10. The provision ofthe inductance 24 in the primary circuit 25 on the one hand, however,and proper adjustment of the control unit 22 on the other make itpossible that the increase of the operating voltage U after the zeropassage occurs later. As it is well known an inductance causes a changeof the phase position of the voltage, in the present embodiment of thevoltage applied to the rectifiers 28 to 31, which effects a retardationof the reincrease of the operating voltage U,,. The ignition of thedischarge occurs now only at point 35, so that a discharge interruptiontime is results instead of the smaller interruption time t, if no suchan inductance is provided.

Although the connection of an inductance in the primary circuit of thetransformer 26 may seem to be an insignificant measure, its effect onthe duration of the ionitriding during a longer lasting ionitridingprocess is considerable. It is achieved, that a plurality of minordefects just forming, and which would remain active after the time havenow sufficiently decayed in order to avoid response of the monitoringdevice 20 and thus avoiding a discharge interruption during a number ofperiods at each response. However, this results in that the responsefrequency of the monitoring device 20 in the starting process of a gasand glow discharge process is substantially reduced, which experiencehas shown to result in a considerable reduction of the total startingperiod required. The discharge interruption time 1 may be adjusted tothe degree of purity of the workpiece surface by means of a suitabledesign of the inductance in combination with the monitoring device 20and the control unit 22.

By way of example, in the operation of an ionitriding furnace with arated power of about 40 kW and a mean operating voltage of 500 V, astarting process of up to 5 hours isfrequently observed. However, if aninductance 24 is included in the primary circuit which is in thisembodiment an iron choke coil having 3, 2 mHy and 20 kVA powerdissipation, the starting process for the same type of workpieces may bereduced to about k to 2 hours. With a normal nitriding period of 6 to 8hours the saving of 2 to 3 hours starting time constitutes a substantialimprovement of the efficiency of the nitriding furnace involved sincethe amount of products to be nitrided is increased by about 25 to 40 percent if the facility is operated continuously. With this power supplyingdevice the interruption time t, is adjusted during the starting processin accordance with the power requirements for heating the workpieces andkept not lower than about 6 or 7 ms. With increasing degrees of purityI, may then be reduced until about t, 2 ms is reached in normaloperating condition. This value should not be negatively exceeded.

Naturally the extension'of the zero passage of the operating voltagedoes not by any means render unnecessary the monitoring device 20 andthe quick switch-off actuated by it. Only the response frequency of thismonitoring device is reduced despite unchanged sensitivity and limitedto cases where the cause of disturbances has not decayed during theextended zero passage of the operating voltage. Despite the raisedefficiency obtained with the power supplying device described, perfectlynitrided surfaces are obtained which are free from so-called dischargepatterns or burning pits.

What is claimed is:

1. In a power supplying device for the operation of a gas dischargecontainer, particularly an ionitriding container, containing metallicworkpieces to be treated, said device being supplied from analternating-current power source, comprising a transformer, the primarycircuit of which is connected to said power source, a rectifying devicecomprising controlled rectifiers, the input of said rectifying devicebeing connected to the secondary circuit of said transformer, the outputof said rectifying means providing direct operating voltage between saidworkpieces and the housing of said container, a monitoring deviceconnected between said workpieces and said container for monitoring saidoperating voltage and the operating current, the output of saidmonitoring device being connected to the input of a regulating andswitching device for said operating voltage, the output of saidregulating and switching device being connected with said rectifyingdevice in combination, the improvement comprising:

a. said rectifying device comprising a fullwave bridge having fourcontrolled rectifier elements,

performing across one bridge diagonal a full wave 5 direct operatingvoltage while the other diagonal of said bridge is connected to theterminals of said secondary circuit of said transformer, pairs ofcontrollingelectrodes of said rectifiers being interconnected andconnected to said output of said regulating and switching device; and

wherein the inductance is an iron choke coil.

1. In a power supplying device for the operation of a gas dischargecontainer, particularly an ionitriding container, containing metallicworkpieces to be treated, said device being supplied from analternating-current power source, comprising a transformer, the primarycircuit of which is connected to said power source, a rectifying devicecomprising controlled rectifiers, the input of said rectifying devicebeing connected to the secondary circuit of said transformer, the outputof said rectifying means providing direct operating voltage between saidworkpieces and the housing of said container, a monitoring deviceconnected between said workpieces and said container for monitoring saidoperating voltage and the operating current, the output of saidmonitoring device being connected to the input of a regulating andswitching device for said operating voltage, the output of saidregulating and switching device being connected with said rectifyingdevice in combination, the improvement comprising: a. said rectifyingdevice comprisIng a full-wave bridge having four controlled rectifierelements, performing across one bridge diagonal a full wave directoperating voltage while the other diagonal of said bridge is connectedto the terminals of said secondary circuit of said transformer, pairs ofcontrolling electrodes of said rectifiers being interconnected andconnected to said output of said regulating and switching device; and b.an inductance connected in series with said primary circuit such thatthe start of each wave of the operating voltage following every zeropassage is retarded by a period of time in the range of 2 - 7milliseconds.
 2. A power supplying device according to claim 1 whereinthe rectifier elements are thyristors.
 3. A power supplying deviceaccording to claim 1 wherein the inductance is an iron choke coil.